Allah tells us about the disbelief, rebellion, oppression and denial of Fir`awn
He tells;
قَالَ فِرْعَوْنُ وَمَا رَبُّ الْعَالَمِينَ
(Fir`awn) said;"And what is the Lord of the `Alamin!"
This is because he used to say to his people;
مَا عَلِمْتُ لَكُمْ مِّنْ إِلَـهٍ غَيْرِى
I know not that you have a god other than me. (28;38)
فَاسْتَخَفَّ قَوْمَهُ فَأَطَاعُوهُ
Thus he fooled his people, and they obeyed him. (43;54)
They used to deny the Creator, may He be glorified, and they believed that they had no other lord than Fir`awn.
When Musa said to them;"I am the Messenger of the Lord of the worlds," Fir`awn said to him, "Who is this who you are claiming is the Lord of Al-`Alamin other than me!"
This is how it was interpreted by the scholars of the Salaf and the Imams of later generations.
As-Suddi said,
"This Ayah is like the Ayah,
قَالَ فَمَن رَّبُّكُمَا يمُوسَى
قَالَ رَبُّنَا الَّذِى أَعْطَى كُلَّ شَىءٍ خَلْقَهُ ثُمَّ هَدَى
(Fir`awn) said;"Who then, O Musa, is the Lord of you two!"
He said;"Our Lord is He Who gave to each thing its form and nature, then guided it aright." (20;49-50)
Those among the philosophers and others who claimed that this was a question about the nature or substance (of Allah) are mistaken.
Fir`awn did not believe in the Creator in the first place, so he was in no position to ask about the nature of the Creator; he denied that the Creator existed at all, as is apparent from the meaning, even though proof and evidence had been established against him.
When Fir`awn asked him about the Lord of Al-`Alamin, Musa said